Since its invention centuries ago, light microscopy has evolved through many incarnations with distinct contrast mechanisms and hardware implementations. However, the fundamental motivation for its use has remained the same—it can resolve features that are not distinguishable by the naked eye. As a result, the push for higher resolution has been the focus of light microscopy development in recent years and several methods have been demonstrated to break the diffraction limit of conventional light microscopy. Despite all these efforts, one often underappreciated fact remains: for many biological samples, diffraction-limited resolution is rarely achieved, even for high-end research microscopes. Ideal imaging performance of a light microscope requires the excitation and/or emission light to pass through samples with optical properties identical to those of the designed immersion media, and any deviation from such conditions causes optical distortions, known as aberrations, leading to the loss of signal, image fidelity, and resolution. In practice, biological samples have inhomogeneous optical properties, so that images are increasingly degraded with increasing depth within biological tissues.
Similar challenges exist for optical telescopes used in astronomy. Light captured from remote stars must first traverse the earth's turbulent atmosphere, which imparts optical distortions that severely degrade image quality. Methods that actively correct for such distortions, known collectively as adaptive optics (“AO”), have evolved to allow ground-based telescopes to obtain diffraction-limited images of extraterrestrial objects. Adaptive optics in astronomy is conceptually simple: a sensor placed near the imaging plane measures the distorted wavefront directly, and an active optical element, such as a deformable mirror, modifies this wavefront in a feedback loop to recover diffraction-limited performance. However, adaptive optics in microscopy is made less straightforward by the difficulty in measuring the aberrated wavefront directly—after all, it is rarely possible to place a wavefront sensor within the specimen. Backscattered light from the specimen has been used for such direct wavefront sensing, but such methods convolve the possibly differing aberrations both to and from the image plane, and are further complicated by multiply-scattered light.